This invention relates to the field of ceramic fibers and other types of ceramic filaments, and to machines, methods and compositions of matter for making same.
Ceramic filaments and fibers are of current interest for a number of different applications, including the reinforcement of structural members made from a variety of materials, filtration media, fiber optic structures, etc. However, primarily because of their inherent characteristics, ceramic materials of low viscosity when melted have not been susceptible to being rendered into high strength long fibers or filaments (i.e., having large length to diameter ratios) of substantially round, uniform, and small diameter throughout, with smooth outer surfaces. This is in contrast with known techniques for making metal fibers which have proven to be more successful.
Thus, it is known to make metal fibers by a number of different methods and apparatus. For example, it is known that metals may be cast into water to form redraw stock which may then be drawn down to finished size, and to draw liquid metal contained in a pyrex tube. It is also known to use the so-called "pendant drop" approach in which drops of molten metal are dropped onto the rim of a revolving wheel. Other approaches include the so-called "melt extraction" techniques which generally are directed to the concept of using a revolving wheel to remove a continuum from the top surface of a source of molten metal. In connection with the foregoing approaches, reference is made to the following: Maringer et al U.S. Pat. No. 3,871,439; Strom-Olsen et al U.S. Pat. Nos. 5,027,886, 5,003,291, 5,015,992, and 5,015,993; Materials Science and Engineering, A133 (1991) 158-61; Journal of Applied Physical, 69 (8), 15 April 1991, 5017-19; Journal of Magnetism and Magnetic Materials, 114 (1992) MMM01436, 1-10; IEEE Transactions on Magnetics, Vol. 28, No. 4, July 1992, 1899-1993; and Journal of Materials Research, Vol. 7, No. 9, September 1992, 2365-72; and the references cited in the foregoing.
While acceptable in many regards for the production of metallic continuums, none of these approaches has proven applicable, or even transferable, to the production of continuums of ceramic, particularly when made from ceramics which are of low viscosity when melted, in the form of long, smooth surfaced fibers or filaments of substantially round, uniform, and small diameter throughout. There are several reasons for this. Among them are that ceramics inherently do not wet non-ceramic surfaces readily and because ceramics lack good thermal conductivity, anything analogous to the extractor wheel used in melt extraction techniques usually are made from non-ceramic materials. Ceramics exhibit viscosities when molten that are too low to permit formation into fibers or filaments by drawing techniques but are too high for them to be formed by melt extraction techniques. Further, they are not susceptible to being rendered molten by such heating techniques as induction or RF heating, and are not reactive, even in the molten state, to electromagnetic fields.
For those and other technical reasons, different kinds of approaches have had to be used in attempting to produce ceramic fibers. In one approach, ceramic is suspended in a gel which is then extruded or otherwise formed into a desired shape which then is heat treated to drive off the non-ceramic constituents and to cause the ceramic to fuse by sintering. In another approach, "grown" fibers of quartz, for example, are drawn through a hot quartz bull block. Another method is to vapor deposit ceramic, such as silicon carbide, onto a base filament, such as a carbon thread. Characteristically, all of these approaches are costly and time consuming, technically difficult to carry out, and, in the end, productive of materials which, in one respect or another, fall short of the desired constellation of quality and characteristics attributes. More specifically, they are often not productive of long filaments which are uniformly of small, round and uniform diameter with smooth surface throughout and having desired high mechanical strength. Their resulting ceramic products, even to the extent that they are, or may be used in, products which are commercially useful, are not of the desired levels of product quality and choice, and are technically and economically prohibited for most applications other than those where their shortcomings can be accommodated, their high costs justified, and no other more viable alternatives are available.
Accordingly, there is interest in easing some of production difficulties and costs that are inherent in these approaches to making ceramic fibers by eliminating some or all of the intermediate and support steps and adjunct sacrificial materials used in them. Thus, other efforts have been directed toward the direct, single step production of finished filaments having such desired characteristics. One approach is that of attempting to adapt the so-called "pendant drop melt extraction" technique to the production of ceramic fibers as disclosed in Schwartzkopf U.S. Pat. Nos. 4,970,194 and 5,053,384 wherein the raw material for the fibers is gravity-dropped onto the edge of a spinning wheel where it is "frozen". That approach purportedly is applicable to produce fibers from certain metal oxide ceramics such as super conductivity alloys. However it requires the use of an oxidizing flame as the melting heat source, and according to the teachings of those patents, it forms short fibers which vary in length from 2-4 cm. and in "diameter" from 20 to 30 micrometers which are only in some occasions circular in cross-section but in most instances are somewhat ribbonlike. In other words, their length to diameter rations are not high, they are not of small, uniform diameter or uniformly circular in cross-section, and their surfaces are not uniformly smooth.
Attempts to utilize something analogous to the previously described melt extraction technique have not been successful, either, to produce amorphous ceramic filaments with small, uniform diameters, uniformly circular cross-section, in long lengths, with smooth surfaces. In that approach, exemplified in Bosswell et al U.S. Pat. No. 5,067,554, a pool of molten material is created with the rim of a cooled, revolving disk inserted into the molten material, so that powder granules or ribbons of solidified ceramic are produced which, in addition to not being fibers or filaments, do not meet the other criteria of desired characteristics hereinbefore described.
Further, as to all of these previous attempts to produce ceramic fibers, the resulting end products are basically of crystalline structure, rather than being amorphous or of small crystals or both as may be desired for improved physical characteristics.
Accordingly, it is an object if this invention to produce ceramic fibers and other filaments.
Another object of this invention is to produce such ceramic fibers and other filaments that are characterized by being of small diameter.
Yet another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by being of substantially uniform diameter.
Still another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by being in long lengths, i.e., are of large length to diameter ratio.
Another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by having surfaces which are substantially entirely smooth.
Yet another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by being of substantially uniform circular cross-section.
Still another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by being of amorphous structure, at least in part.
Another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by having surfaces which are characterized by their great flexibility.
Yet another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by having been free-formed.
Still another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by being high strength.
Another object of this invention is to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives and are characterized by being of small crystalline structure, at least in part.
Still another object of this invention is to provide methods to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives.
Yet another object of this invention is to provide apparatus to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives.
Another object of this invention is to provide compositions of matter to produce ceramic fibers and other filaments that meet one or more of the foregoing objectives.